14.1.1 Inserting ‘@’ with @@ and @atchar{}

@@ produces a single ‘@’ character in the output. Do
not put braces after an @@ command.

@atchar{} also produces a single ‘@’ character in the
output. It does need following braces, as usual for alphabetic
commands. In inline conditionals (see Inline Conditionals), it
can be necessary to avoid using the literal ‘@’ character in the
source (and may be clearer in other contexts).